The thermal decomposition of benzoic acid to benzene and carbon dioxide begins non-catalytically at 370.degree. C. in a glass bulb and is substantially complete at 400.degree. C. (Chemical Abstracts, vol. 41, 646) according to the original article of Wolfgang Mosher in Helv. Chem. Acta. 14, 971-97 (1931) and such dissociation is accelerated by copper and cadmium catalysts. Said dissociation occurs at temperatures as low as 245.degree. to 250.degree. C. in the presence of Zn-Cu-Cr oxide-type catalysts according to Corliss R. Kinney and David P. Langlois in J. Am. Chem. Soc., vol. 53, 2189-2192 (1931). Decarboxylation of benzaldehyde to high yields of benzene is aided by plasma of glow discharge according to Published Patent Application ("Offenlegungsschrift") No. 2,038,272 of the Federal German Republic, published Mar. 16, 1972. According to British Pat. No. 735,300, published Aug. 17, 1955, toluic acids heated to 400.degree. C. in the presence of chromites of Zn, Cd, Zn-Cd, Zn-Fe or ZnO with either CuO or CdO are converted to toluene.
From the state of the art at the time of making the present invention it appears that the main interest in decarboxylation of benzene carboxylic acids was to prepare a higher quality benzene carboxylic acid of lesser COOH group content from a benzene carboxylic acid of higher COOH group content and lower quality such as a coal acid or to obtain a benzene carboxylic acid of exceptionally high quality; e.g., pharmaceutical quality benzoic acid, from phthalic anhydride by converting it to o-phthalic acid and decarboxylating it. But there was no apparent interest in the decarboxylation of benzene carboxylic acids to aromatic hydrocarbons.
In an altogether different environment a new problem has arisen. In the commercial manufacture of benzene di- or tricarboxylic acids (e.g., isophthalic acid (IPA), terephthalic acid (TA) or trimellitic acid (TMLA)) there is obtained, after maximizing recovery of such acid and recovery for reuse the reaction solvent, a residue which is a mixture of oxygen-containing derivatives of benzene and toluene which are mono-, di- and tricarboxylic acids, aldehydocarboxylic acids, and methylol-substituted benzene or toluene or their carboxylic (benzoic or toluic) acids and which also contains components of catalysis. Usually such components of catalysis are Co-Mn-Br or Co-Mn-Ce-Br from liquid phase oxidation of a xylene or pseudocumene (1,2,4-trimethylbenzene) with air in the presence of acetic acid reaction solvent. A similar residue is also obtained from the neat oxidation of liquid o-xylene with air in the presence of Co-Mn-Br catalyst system after dehydrating the o-phthalic acid formed to its anhydride under conditions which vaporize the anhydride, water and materials boiling between the anhydride and water. While such residues amount to from 2 to 25 weight percent of the benzene di- or tricarboxylic acid produced, such residue production annually is substantial in view of the millions of kilograms of the benzene di- or tricarboxylic acids produced annually.
Such residues contain water soluble benzene carboxylic acids and water soluble forms of the components of catalysis. Landfill disposal of such residues is undesirable because rain and ground water leach out those carboxylic acids and soluble forms of the components of catalysis and can contaminate surface run off water and eventually streams as well as below surface aquafiers. Disposal of such residues can be made by incineration as disclosed in U.S. Pat. Nos. 4,258,227 and 4,266,084 and use made of the resultant heat produced, but the catalyst components are converted to forms in the resultant ash which are difficult and/or expensive to convert to reusable forms for the oxidation of the methyl-substituted benzenes. Although in such residues the substituted benzene and toluene compounds whose substituents are the carboxy-, aldehyde- and methylol substituents are individually desirable and useful commercial products, it is not economically feasible to separate and recover the individual compounds from the residues.
Based on the knowledge that most of the oxygen-containing aromatic compounds in the residue can be decarboxylated by thermal means, it would be desirable to devise a decarboxylation process which would convert the oxygen-containing aromatic compounds to aromatic hydrocarbons which are volatile under such process conditions so that the hydrocarbon vapors can be readily removed and condensed for their recovery. It is also known that under the severe thermal conditions required for substantially complete decarboxylation to convert the oxygen-containing substituted aromatics to benzene and toluene there can also occur ring coupling (e.g., to form biphenyl) and ring fusion as well as charring of some of the organic compounds.
To use a decarboxylation catalyst for the thermal conversion of the foregoing residues to easily recoverable and useful aromatic hydrocarbons would be desirable providing the use of catalyst does enhance the production of the aromatic hydrocarbons but does not make useless the resulting char or further contaminate the catalyst components present so as to make the recovery of cobalt, the most expensive component, technically and commercially unattractive.
We have in our laboratories investigated the use of various compositions previously suggested as decarboxylation catalyst and found the resulting thermal conversions to be unattractive.
The consistently better prior suggested catalyst was found to be the combination of zinc oxide and alumina. Such catalyst is used at 500.degree. C. with one to two gram samples of terephthalic acid process residues introduced consecutively at about 5-minute intervals into the heated quartz tube containing said catalyst over a five-day period. The liquid aromatic hydrocarbon yield decreases from 26.4 weight percent down to 14.6 weight percent of residue fed over the five-day period and considerable blackening of the catalyst is observed. Said liquid aromatic hydrocarbon decrease occurs with a residue-to-catalyst weight ratio of no more than about 2:1. Such results indicate a very short life for the ZnO-- alumina catalyst and that frequent regeneration thereof would be necessary for such catalyst to be used commercially.
Such short useful catalyst life made the use thereof for pyrolysis of the aforementioned residues commercially unattractive. However, the present inventive non-catalytic decarboxylation of the oxygen-containing derivatives of benzene and toluene and especially the cyclic ester, methylol-, aldehyde-, carboxy-, carboxy- and aldehyde-, keto- and carboxy-, and methylol- and carboxy-substituted benzenes obtained as a residue from the manufacture of benzene di- and tricarboxylic acids does not have such disability and is technically and commercially attractive.